I don't think that we can. While we expect they rather keep their business running than stealing people's money, we cannot know if they indeed play fair and go random, or generate their numbers by a rule. But...

In theory if they do this, the seeds the devices will generate will be a rather small subset of all the possible seeds (so they can brute force them in a reasonable amount of time).
However, if this would happen, there's a very big chance collision would happen, and rather often, between different users' seeds/wallets. If this happens, most do post about it (especially if it's them losing money).
Yes, I know, if this happens, people tend to point to hacks and unsafe storage of the keys, but still.. there would be much more of those imho.
Also I'd expect some users be nice and, instead of stealing, start writing about it.

Imho for now it's pretty safe to assume they did play fair until now.